Someone asked us for the birthday data, and Aki replied:

We used 1969-1989 also in BDA3 https://users.aalto.fi/~ave/BDA3.pdf
And there we mention that the birthday data come from the National Vital Statistics System natality data and are at http://www.mechanicalkern.com/static/birthdates-1968-1988.csv, provided by Robert Kern using Google BigQuery.

The code for the BDA3 example is at https://research.cs.aalto.fi/pml/software/gpstuff/demo_births.shtml (with link to data, too)

We have also a more recent paper using the data https://arxiv.org/abs/2004.11408 with R and Stan code available at https://github.com/gabriuma/basis_functions_approach_to_GP and the copy of the data is in the same repo at
https://github.com/gabriuma/basis_functions_approach_to_GP/tree/master/uni_dimensional/birthday-dataset